This invention is designed to improve image quality of a projection optics lithography system (e.g. of the stepper and/or scanner type).
In a projection optics lithography system the image in photoresist is a function of the illumination source shape (or distribution), the object amplitude (the mask [or reticle] features), the lens attributes or pupil distribution, and the resist properties. The equation below states this in a more rigorous fashion

The “carrotted” variables in the above equation refer to sets of coordinate. Hence {circumflex over (x)}=(x,y) and z are the image space coordinates; {circumflex over (α)}=(α,β) and {circumflex over (α)}s are the pupil space coordinates. In the current use of source-mask optimization (SMO), the image, I, is the desired target. The source shape, J, and the Fourier transform of the Mask Spectrum are variables that are optimized. The Pupil and the Film (resist) are assumed fixed. SMO typically seeks to solve the inverse of this equation.